In flexible packaging applications, polymeric films and/or paper webs are often combined to a metallic layer generally consisting of aluminium. This metallic layer can be a self-supporting foil, typically between 6 and 15 μm thick, or it can be a much thinner layer, generally below 0,1 μm thick, on a polymeric or paper support. This metallic layer is usually applied by a vacuum coating process, in which vaporised metal atoms adhere to a suitable substrate. This vacuum metallization process is extensively described in the literature.
Metal foils and metallic coatings have several functions, including barrier functions with regard to atmospheric gases, water vapour, radiation, etc. and, in addition, play an important role in the marketing aspects of a package. Such metallic layers give a particular brilliance and colour intensity to the overlying printed design, and, where visible by themselves as a metallic design element, give a perception of quality and protection of the package contents. In many cases though, when the barrier needs of the package allow it, the producer would wish to combine these positive marketing aspects of a metallic layer with a partial window in the metallic layer. In the case of transparent polymeric films the main purpose would be to allow for visual inspection of the packaged product by the consumer in the retail phase. In the case of multilayer structures involving paper or other non-transparent substrates, there might be other functional or marketing advantages in having a partial window in the metallic layer.
In most of the following, we focus on the case of transparent polymeric film laminates with thin metallic coatings as being the most important class of multilayer materials in which the current invention could be applied. Here the current industrial practice for obtaining a partial demetallization has been a procedure involving the following processing steps:    a) a printing step, involving a metallized film, typically consisting of an oriented coextruded polypropylene film, between 15 and 30 μm thick and vacuum coated with a layer of aluminium, about 100 to 1000 Å thick, which is partially printed on a regular printing line (typically a gravure or flexo press) using a suitable ink system and an overlacquer to protect the inks during subsequent processing. In most cases, a primer is applied between the metallized layer and the printing inks to improve adhesion. When this printed film is intended for partial demetallization, care is taken that neither primers nor inks or overlacquers cover the aluminium in the area to be demetallized. In the case that an unprinted metallized film is intended to be partially demetallized, only the protective overlacquer would need to be printed, possibly with the addition of a suitable primer;    b) a demetallization step, involving the passage of the film prepared according to step (a) through a concentrated sodium hydroxide (NaOH) solution in water, whereby the exposed portions of the metallic aluminium are dissolved and the dissolved metal is subsequently washed away with water, followed by a drying operation to remove excess moisture;    c) a lamination step, whereby the printed demetallized film is taken on a laminating machine and bonded to another self-supporting film web, typically 15-30 μm thick, using a suitable adhesive system (most often a two-component polyurethane adhesive).
The procedure described above and in practical use today is seen to involve at least three separate converting steps, which makes it a very costly process, limiting its market penetration to high-end products. A further disadvantage is the time loss because of the logistics of the three-step process, especially if converting and demetallization equipment are found in different production sites. A further disadvantage is the fact that particular in-line operations, such as the application of a cold seal lacquer on the backside of the metallized film, become impossible because of the various processing steps. A further disadvantage is the lack of an optimal quality control in the printing step, since the final result only becomes visible after the demetallization step.
State of the Art
The above multi-step procedure being the current industrial practice, we believe that the following documents represent the closest prior art.
U.S. Pat. No. 5,628,921 describes a process for carrying out the classical demetallization involving a caustic solution and a washing step, in-line with a gravure printing operation, through the use of a dedicated machinery custom made for this purpose and essentially consisting of a classical demetallization equipment connected to a classical gravure printing press. It would seem that this process and equipment has the advantageous possibility of in-line quality control checking the demetallized area in respect of the printed design, this is however achieved at the expense of a much higher investment cost for this complicated machinery.
U.S. Pat. No. 3,647,508 discloses a process for carrying out the demetallization whereby the etching agent is mixed with a film-forming dispersion thereby achieving that the etching agent can be contained within a dried coating remaining on the web. However this method only claims particular effects on the conductivity, reflectivity and adhesion of the final product, not transparency, and an optional washing step is described evidently for this purpose.
The purpose of the present invention is to obtain clarity and transparency (high transmission and clarity and low haze) of the demetallized window, which still requires a washing step in the prior art.
In summary, neither of the two described processes constitutes a significant breakthrough versus the current practice described in the technological background.
Aims of the Invention
The present invention aims to provide a simplified process for partial demetallization of flexible substrates, performed on standard equipment such as a gravure or flexo press, rather than on machinery specifically designed for demetallization. Furthermore, this invention aims to reduce complexity and cost of the entire process by performing said process in-line with other converting operations such as printing, laminating and/or coating in one continuous operation.